Variable light transmitting filter for cameras

ABSTRACT

A filter for permanent installation to extend the effective range of iris openings of a camera; the filter has light transmittance characteristics varying smoothly from low transmittance at the center to high transmittance at the periphery.

OR 3 9 7 O 9 3 l 4 United States Patent 1 3,700,314 Busby, Jr. 1 Oct.24, 1972 [54] VARIABLE LIGHT TRANSMITTING [56] References Cited FILTERFOR CAMERAS 72 I Ed 5. Bush J M l P k UNITED STATES PATENTS t I u s 1 rat 3,584,558 6/1971 Easterly ..95/64 R 3,006,265 10/1961 La Rue..350/3l4 X u Ampex Corporation, R od Clty, 3,035,503 5/1962 Lee Calif.

Primary Examiner-David Schonberg [22] Flled' April 1971 AssistantExaminer-Toby H. Kusmer [211 App]. No.: 131,996 Attorney-Robert G. Clay[52] US. Cl. .350/314, /64 R, 95/64 D, [57] ABSTRACT 350/315 A filterfor permanent installation to extend the effec- [51] Int. Cl. ..G02b5/22 tive range of iris openings of a camera; the filter has [5 8] Fieldof Search ..350/3l4, 315; 95/64 R, 64 D light. transmittancecharacteristics varying smoothly from low transmittance at the center tohigh transmittance at the periphery.

3 Claims, 2 Drawing Figures Patented Oct. 24 1972 T II3 1 INVENTOR.EDWIN S. BUSBY ATTORNEY VARIABLE LIGHT TRANSMITTING FILTER FOR CAMERASBACKGROUND OF THE INVENTION This invention relates to optical systemsfor cameras, and particularly to light filters therefor.

Previously in the television camera art, it has been difficult to dealwith live scenes having widely varying brightness. As an illustrativeexample, suppose the camera is to be focused upon an indoor sceneincluding a subject which then moves to the outdoors in bright sunlight.It will be understood that the range of input brightness that a giventelevision system can accommodate is limited, and that the range of irisopenings that can be built into a lens of given size is predetermined bymechanical limitations. The system is usually arranged so that the rangeof iris openings, from fully open to closed, does not cover the fullrange of input brightness i.e., from indoors to outdoors, but is chosento permit good picture contrast only for darker scenes. To provide goodcontrast in brighter scenes, the prior art normally uses a system ofindividual light filters, each of which has a different transmittance.Working indoors without a filter then, an iris opening may be chosen tocorrespond only with the indoor scene illumination. When the subjectmoves outdoors, a dark filter is inserted into the optical system topermit the use of an attainable iris opening.

The difficulty with this prior art method of inserting light filters atdiscrete points in the brightness range is that the effect on the vieweris abrupt and disconcerting, and the iris must always be re-adjustedafter the filter is inserted or removed.

Accordingly, it is an object of the present invention to provide a lightfilter for extending the iris opening range of a lens in a smooth andunobtrusive manner not noticeable by the viewer. Other objects andadvantages will be made clear in connection with the followingdescription.

SUMMARY OF THE INVENTION A filter for permanent installation to extendthe effective range of iris openings of a camera; the filter has lighttransmittance characteristics varying smoothly from low transmittance atthe center to high transmittance at the periphery.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a broken-away view of an iris andfilter system incorporating the invention; and

FIG. 2 is a graph illustrating the construction of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS is capable of, and the centralzone 14 has a gaussian or an exponential distribution'of transmittancevaryingradially from minimum value at the precise center of the filterto maximum value at the boundary 13, this maximum value being equal tothat of the peripheral zone 16, so that the central zone 14 blendssmoothly therewith.

With a filter as above described, the total transmittance with the irisfully open as shown can be established at a value such that thetelevision picture tube gives a picture of satisfactory contrast whenthe scene being viewed by the camera is generally dark, i.e., darkbackground and dark objects, while yet the contrast is also satisfactorywhen the scene consists of bright objects and bright background, butwith the iris in its most fully closed down position, exposing only thedarkest central area of the portion 14. Thus the same effect can becreated with only one filter, as is provided in the prior art by the useof several. Furthennore the transition from one filter condition toanother is made smoothly and unobtrusively by means of the presentfilter, so that the monitor viewer is not disconcerted, andcounteradaptive re-positioning of the iris with each filter change is nolonger needed.

The variable-transmission portion 14 of the filter may be made in any ofa number of suitable ways. An actual filter has been satisfactorily madeby sputtering techniques, to fit a transmission curve similar to that ofFIG. 2 described below. Other ways include the exposure of photographicfilm to light beams of varying cross-sectional intensifies. A laser beamhavinga gaussian cross-sectional intensity distribution may be used forexample. The illustration of FIG. 1 may be considered to be an actualexample of a filter according to the invention if the drawing paper isheld up to pass light. This illustration was produced by superimposingdiscrete rings of dotted illustrators shading material of difi'erent dotdensities, according to the curve of FIG. 2, and then photographing thecomposite through a defocused camera lens so as to diifuse and smoothout the transitions between rings.

As explained in part above, FIG. 2 shows an exemplary curve 17 plottedfrom the equation T M, wherein the term R is the radius of variouspoints within the area 14 and the term T represents thecorrespondingtransmittances of such points. Such a relationship has beenfound to be satisfactory in actual practice, and many others may bepossible.

What is claimed is: l. A variable light-transmitting filter combination,comprising: I

a mask having a variable area aperture formed therein and defined inpart by a camera type circumferentially expandable and contractable irismechanism; and

a semi-transparent filter element covering said aperture;

said filter element having a smoothly varying transmittancecharacteristic from point to point thereof;

wherein said transmittance characteristic of said filter varies from apoint of minimum light transmittance at the center of said filter andiris and increases smoothly radially outwardly for at least a centralportion of the filter.

2. The combination recited in claim l wherein the radially outwardlyincreasing portions of said filter varies in accordance with theequation T -R", in which T is the transmittance expressed in percent ofincident light passed, R is the radius from the center of the filter,and K is a chosen exponential value.

3. The combination recited in claim 2, wherein K is equal to 0.72.

1. A variable light-transmitting filter combination, comprising: a maskhaving a variable area aperture formed therein and defined in part by acamera type circumferentially expandable and contractable irismechanism; and a semi-transparent filter element covering said aperture;said filter element having a smoothly varying transmittancecharacteristic from point to point thereof; wherein said transmittancecharacteristic of said filter varies from a point of minimum lighttransmittance at the center of said filter and iris and increasessmoothly radially outwardly for at least a central portion of thefilter.
 2. The combination recited in claim 1, wherein the radiallyoutwardly increasing portions of said filter varies in accordance withthe equation T RK, in which T is the transmittance expressed in percentof incident light passed, R is the radius from the center of the filter,and K is a chosen exponential value.
 3. The combination recited in claim2, wherein K is equal to 0.72.